As an internal material which is used as an internal material in the inside of a shoe, for example, there have hitherto been known materials resulting from sticking a canvas onto wool and materials mainly composed of polyurethane foam (see, for example, Patent Documents 1 and 2). However, when a shoe insole is configured of such an internal material of shoe, there was involved such a problem that in using the shoe insole, the weight is heavy and the air permeability is so bad that a stuffy feeling is produced. Also, in materials using a natural fiber such as wool, there was involved such a problem that when washed with water, the cushioning properties are lowered.
For that reason, there have been demanded proposals of an internal material of shoe having excellent lightweight properties, cushioning properties and air permeability. Also, there have been demanded proposals of an internal material of shoe also having heat retaining properties to be used in the winter season or low-temperature environment.
Incidentally, as fiber products having heat retaining properties, there have hitherto been proposed a material using an acrylate based hygroscopic and exothermic fiber (see, for example, Patent Documents 3 and 4), an internal material of shoe having a hygroscopic and exothermic organic fine particle attached thereto (see, for example, Patent Document 5).
[Patent Document 1] Registered Utility Model No. 3066533
[Patent Document 2] JP-A-6-14190
[Patent Document 3] JP-A-2000-265365
[Patent Document 4] JP-A-2001-112578
[Patent Document 5] JP-A-2003-105657